Conventionally, computed tomography (CT) apparatuses that reconstruct a tomographic image of a test object have been in practical use in the medical field and non-destructive testing, for example. An X-ray CT apparatus, which is a CT apparatus, irradiates a test object with an X-ray in all directions of 360 degrees therearound, and detects the X-ray passing through the test object in each direction, for example. The X-ray CT apparatus then uses X-ray projection data obtained by performing logarithmic transformation and the like on X-ray detection data to reconstruct a tomographic image.
To obtain an accurate tomographic image, it is necessary to change the irradiation angle of the X-ray with high granularity and to use an X-ray detector in which X-ray detecting elements are arranged in high density. As a result, the resolution of the X-ray detection data and the number of images captured per unit time increase. To address these needs, there has been developed a technology for reducing costs for accumulation and transmission of X-ray detection data by compressing the X-ray detection data.
If X-ray detection data is compressed, the X-ray detection data thus compressed is expanded and then transformed logarithmically to generate X-ray projection data used for reconstruction of a tomographic image. In the data obtained by logarithmically transforming the data thus expanded, however, errors increase compared with data obtained by logarithmically transforming the X-ray detection data, resulting in degraded quality of the tomographic image.